1. Field of Invention
The present invention relates to an installation for fabricating a photomask. More particularly, the present invention relates to an installation that can concurrently produce separate patterns, one on each side of a photomask.
2. Description of Related Art
Photolithography is one of the major techniques used for transferring electric circuit patterns onto a wafer. FIG. 1A is a cross-sectional view showing a single-sided photomask and the process of transferring a pattern onto a wafer. As shown in FIG. 1A, the main body 10 of the photomask is constructed from a planar transparent substrate. The main body 10 can be made from glass or quartz, and the various patterned layers on a semiconductor wafer are obtained by coating a masking layer 12 having the required pattern over the transparent substrate 10. The masking layer 12 can be a layer of metallic chromium (Cr).
Furthermore, an anti-reflection coating 14 is often coated on top of the masking layer 12 to lower the amount of reflection during light exposure. By illuminating a photoresist layer on top of a semiconductor substrate 16 through a photomask, the pattern on the photomask is transferred to the photoresist layer. After the photoresist layer is chemically developed, a patterned photoresist layer 18 is formed over the substrate 16. As the level of integration for integrated circuit devices increases, the dimensions of a device and its line width gradually shrink.
Consequently, dimensions of pattern formed in a photomask must be carefully controlled to fall within much narrower limits in order to have a high processing yield. Since pattern dimensions on a photomask must be precisely controlled, the level of difficulty in producing a high-quality mask as well as the extent of limitations increases considerably.
FIG. 1B is a cross-sectional view showing a double-sided photomask and the process of transferring a pattern to a wafer. As shown in FIG. 1B, a required pattern on a wafer 26 is alternately extracted and placed on respective faces of a transparent substrate 20. By spreading out the density of pattern on a photomask, the level of difficulty and the number of limitations in the production of photomask are correspondingly reduced. Moreover, critical dimensions can be more precisely controlled, thereby increasing the resolution of photolithographic operation. The main body 20 of this double-sided photomask has two patterns on respective surfaces 22a and 22b.
However, through proper illumination, patterns on these two surfaces can be combined to form the necessary pattern for forming a device pattern on the photoresist layer 28. Similarly, it is common for a double-sided mask to have anti-reflection coatings 24a and 24b coated on top of the patterned layers 22a and 22b, respectively.
Since a double-sided photomask has the advantage of lowering pattern density compared with a singled-sided mask, a photoresist layer can be patterned more accurately. In light of the foregoing, there is a need to provide an installation that can produce double-sided photomask quickly and efficiently.